The present invention concerns solar cells. In particular, the present invention is a solar cell whose absorption of incident solar light is enhanced in a range of light wavelengths where the intrinsic absorption by the semiconductor is especially weak by means of a two-dimensional reflecting diffraction grating.
Many of the materials used for solar cells, such as silicon, require a thick active layer because of low absorption of incident light. In particular, silicon has low solar absorptance in the infrared and far-infrared. However, such thickness requirements impose severe purity constraints on the material. This follows because the purity of the active material determines the lifetime of the electron-hole pair that is generated by the absorbed sunlight. The lifetime of the generated pair is proportional to the square of diffusion length. The diffusion length is the average length a photo-generated carrier can travel before it recombines. Since the cell must be thick to allow for absorption of the incident sunlight, the diffusion length must be of the order of the thickness of the cell to allow for the generated pair to contribute to the electric current generated by the cell. This, in turn, requires high purity active material.
It is desirable to reduce the thickness of the active material for several reasons. First a thin layer would reduce the amount and, therefore, the cost of the active material. Second, a thin layer would allow for a reduction in the diffusion length of the generated carriers. Shorter diffusion lengths allow for a reduction in the purity of the active material as discussed above.